1. Field of the Invention
The present invention relates to a film heating type image heating device which brings a heat-resistant film into sliding contact with a heater which generates heat upon energization, brings a member to be heated into tight contact with a surface, opposite to the heater, of the film, and passes the member to be heated together with the film at the position of the heater, thereby applying heat energy from the heater to the member to be heated via the film, and a heater used in the image heating device.
2. Related Background Art
As the above-mentioned film heating type heating device, U.S. Pat. No. 5,149,941, U.S. patent application Ser. No. 444,802, and the like previously proposed by the present applicant are known. Such a heating device can be utilized as an image heating/fixing device for an image forming apparatus such as an electrophotographic copying machine, a printer, a facsimile apparatus, or the like, i.e., an image heating/fixing device for heating and fixing a non-fixed image visualizing agent (toner) image corresponding to target image information and formed by a direct or indirect (transfer) method on the surface of a recording member (an electro-facsimile sheet, an electrostatic recording sheet, a transfer medium sheet, a print sheet, or the like) using a toner consisting of a hot-melt resin by image forming process means such as electrophotography means, electrostatic recording means, magnetic recording means, or the like.
For example, the heating device can also be used as a device for improving the surface property such as gloss by heating a recording member which carries an image, a device for temporarily fixing an image on a recording member, or the like.
More specifically, the film heating type image heating device comprises a thin heat-resistant film (sheet), movement driving means for the film, a heater which is fixed and supported on one surface side of the film, and a pressing member arranged to oppose the heater on the other surface side of the film, for bringing an image visualizing agent (toner) image carrying surface of a recording member on which an image is to be fixed into contact with the heater via the film. The image heading device operates based on the following principle. That is, at least during execution of image fixing processing, the film is fed at the same speed and the same direction as those of a recording member, which is fed between the film and the pressing member, and is subjected to the image fixing processing, and the recording member is caused to pass a fixing nip portion as a fixing portion defined by a press contact state between the heater and the pressing member to sandwich the fed film therebetween. Thus, the toner image carrying surface of the recording member is heated by the heater via the film to apply heat energy to a non-fixed toner image, and to soften and melt the toner image. Thereafter, the film and the recording member which have passed the fixing portion are separated at the separation point.
FIG. 13 is a partially cutaway plan view of a heater used in the film heating type fixing device, and a block diagram of an energization control system.
A heater 2 shown in FIG. 13 comprises:
a. an electrically insulating, heat-resistant, and low-heat capacity elongated ceramic substrate 3, which has its longitudinal direction extending in a direction substantially perpendicular to the feeding direction of a heat-resistant film 1, and consists of, e.g., Al.sub.2 O.sub.3 (alumina), AlN, SiC, or the like; PA1 b. an energization heat generating member 4 which is formed into a stripe- or band-shaped pattern along the longitudinal direction of the substrate at the central portion, in the widthwise direction, of one surface (face) of the substrate 3, serves as a heat source, and consists of a silver/palladium alloy (Ag/Pd), or the like; PA1 c. power supply electrodes 5, 6, and 6' formed on the substrate surface to be electrically connected to the two end portions of the energization heating member 4, and through holes 50; PA1 d. an electrically insulating overcoat layer 7 of, e.g., glass serving as a surface protective layer which covers the energization heating member forming surface of the substrate 3; PA1 e. a temperature detection element 8 such as a thermistor and a temperature fuse 9 as a temperature detection element (thermal protector) for a safety countermeasure, which are arranged to be in contact with the other surface side (back side) of the substrate 3; and the like. PA1 a heater; and PA1 a film which is moved while one surface thereof contacts the heater, and the other surface thereof contacts a recording member which supports an image, PA1 the heater comprising a resistor for generating heat upon energization, and an energization electrode arranged to alternately have different polarities in a direction perpendicular to a feeding direction of the recording member. PA1 a resistor for generating heat upon energization; and PA1 an energization electrode arranged to alternately have different polarities in a longitudinal direction of the resistor. PA1 a heater; and PA1 a film which is moved while one surface thereof contacts the heater, and the other surface thereof contacts a recording member which supports an image, PA1 the heater comprising a heating member for generating heat upon energization, an electrode arranged at an end portion, in a longitudinal direction, of the heating member, and an energization path branching from an intermediate portion, in the longitudinal direction, of the heating member, and PA1 the energization path comprising a resistor having a negative temperature--resistance characteristic. PA1 a heating member for generating heat upon energization; PA1 an electrode arranged at an end portion, in a longitudinal direction, of the heating member; and PA1 an energization path branching from an intermediate portion, in the longitudinal direction, of the heating member, PA1 the energization path comprising a resistor having a negative temperature--resistance characteristic. PA1 a heater; and PA1 a film which is moved while one surface thereof contacts the heater, and the other surface thereof contacts a recording member which supports an image, PA1 the heater comprising a heating member for generating heat upon energization, an electrode arranged at an end portion, in a longitudinal direction, of the heating member, and an energization path branching from an intermediate portion, in the longitudinal direction, of the heating member, and PA1 the energization path comprising a switching element which is enabled at a temperature not less than a predetermined temperature. PA1 a heating member for generating heat upon energization; PA1 an electrode arranged at an end portion, in a longitudinal direction, of the heating member; and PA1 an energization path branching from an intermediate portion, in the longitudinal direction, of the heating member, PA1 the energization path comprising a switching element which is enabled at a temperature not less than a predetermined temperature.
The overcoat layer 7 side of the heater 2 corresponds to the film sliding contact surface side, and the heater 2 is fixed and supported by a support portion (not shown) via a heat-insulating heater holder 13 to expose this surface side externally.
The temperature of the heater 2 rises when a voltage is applied from an AC power supply 20 across the power supply electrodes 5 and 6 at the two ends of the energization heating member 4, and the energization heating member 4 generates heat.
The temperature of the heater 2 is detected by the temperature detection element 8 on the back side of the substrate, and the detected information is fed back to an energization control unit 15. The control unit 15 controls energization from the AC power supply 20 to the energization heating member 4 based on the detected information, thereby executing temperature control, so that the temperature of the heater 2 detected by the temperature detection element 8 upon execution of fixing becomes a predetermined temperature (fixing temperature).
The temperature control of the heater 2 is realized by adopting a method of controlling the applied voltage or current to the energization heating member 4, or a method of controlling the energization time. As the method of controlling the energization time, zero-crossing wave number control for controlling energization and non-energization states in units of half cycles of a power supply waveform, and phase control for controlling the phase angle to be energized in units of half cycles of a power supply waveform are known.
More specifically, the output from the temperature detection element (thermistor) 8 is A/D-converted, and is fetched by a CPU. Based on the fetched information, an AC voltage to be applied to the energization heating member 4 is pulse-width-modulated by the phase control, wave number control, or the like by an SSR (solid state relay) having a TRIAC and the like, thereby controlling energization to the energization heating member 4, so that the temperature of the heater detected by the temperature detection element 8 becomes constant.
The temperature fuse 9 is arranged in the vicinity of or to be in contact with the back of the substrate 3 of the heater 2 while being connected in series with the energization path to the energization heating member 4. When the energization control of the energization heating member 4 goes wrong, and the heater 2 causes an abnormal temperature rise (thermal runaway of the heater), the temperature fuse 9 operates to open the energization circuit to the energization heating member 4, thereby disabling energization to the energization heating member.
In the above-mentioned film heating type device, since the heater 2 having a low heat capacity can be used, the wait time can be shortened as compared to a conventional heat roller type heating device (quick start characteristic). In addition, since a quick start is allowed, the above-mentioned device does not require a preheating process when it is in idle, thus attaining savings in total power consumption. Also, the above-mentioned device has a merit capable of solving various drawbacks of devices of other heating types, and is effective.
However, a resistor material used in the energization heating member of the heater is normally a noble metal (e.g., Ag/Pd), and is very expensive.
When such a material is replaced by an inexpensive material to reduce cost, the inexpensive material has a high volume resistance value, and cannot be used in the conventional device.
More specifically, the heater must generate electric power capable of obtaining a predetermined temperature rise or higher within a limited period of time. On the other hand, a power supply voltage to be supplied to the heater is normally a commercial power supply voltage (AC 100/200 V), and is fixed. Therefore, the resistance value of the heater must be equal to or smaller than a predetermined value.
The resistance value of the heater is determined by the thickness, the width (in the feeding direction of a recording member), the length (in a direction perpendicular to the feeding direction of a recording member), and the volume resistance of the energization heating member. The length is almost the same as the width of a recording member, and is fixed. As for the width, when the width is set to be larger than the nip width, it is not effective since heat generated by a portion extending outside the nip is not conducted to a recording member. An increase in thickness of the heating member is limited by a manufacturing method such as screen printing.
More specifically, in order to set the resistance value of the heater to be equal to or smaller than a predetermined value, the volume resistance value of the energization heating member must be set to be a predetermined value or less. For this reason, an inexpensive resistor cannot be used as long as it has a high volume resistance value.
In the above-mentioned film heating system, when a heating/fixing operation is continuously performed using small recording members, a difference between heat dissipation amounts of a portion which contacts the recording member and a portion which does not contact the recording member is generated. More specifically, in an area where a recording member is not fed, the temperatures of the film, the pressing member, and the like become higher than those in an area where the recording member is fed. For this reason, the film, the pressing member, and the like corresponding to a non-paper feeding area thermally deteriorate. This phenomenon is called a non-paper feeding portion temperature rise.